1. Field of the Invention
The present invention relates to a document feeder for automatically feeding documents to a document reader unit of an image-forming apparatus such as electrophotographic copying machine or the like machine, and an image-forming apparatus using the document feeder. More specifically, the invention relates to a document feeder for reading double-sided documents, and an image-forming apparatus using the document feeder.
2. Background Art
As is well known, an image-forming apparatus such as electrophotographic copying machine or the like machine is employing a so-called ADF (automatic document feeder) for automatically feeding documents to a document reader unit. In recent years, it has been desired to develop a document feeder capable of handling double-sided documents having images formed on both surfaces thereof.
There has been proposed a document feeder capable of handling double-sided documents having images formed on both surfaces as exemplified by, for example, a document processing machine disclosed in Japanese Unexamined Patent Publication No. 7-175279. This document processing machine has two modes. In a first mode, a single-sided document is read out and is directly discharged onto a discharge tray. In a second mode, a first surface of a double-sided document is read out and the document is once fed onto a discharge tray and, then, the document is not completely discharged but is switched back on the discharge tray and is fed again onto a document feed passage through a double-side feed passage. A second surface is then read out and the document is fed onto the discharge tray. The document is not completely discharged onto the discharge tray but is switched back again, fed again onto the document feed passage through the double side feed passage and is then discharged and stacked on the discharge tray with the first surface being faced downwards, so that pages are in order when the double-sided documents are successively stacked on the discharge tray. Therefore, the double-sided document is circulated by a roller that effects the discharge and inversion at the inlet of the discharge tray and by the double-side feed passage which feeds the document from the discharge tray onto the document feed passage. To discharge t he double-sided document, the document is inverted by the roller, and is discharged onto the discharge tray by the above-mentioned roller through a passage similar to that for reading the second surface of the document.
There has further been proposed a sheet conveyer device as disclosed in, for example, Japanese Unexamined Patent Publication No. 8-133551 (Japanese Patent Application No. 6-298816). According to this technology, after the second surface is read out, the double-sided document is inverted (switched back) by an inversion unit provided on a place deviated from the discharge tray and is discharged onto the discharge tray so as to be stacked with the first surface being faced downwards, so that pages are in order when the double-sided documents are successively stacked on the discharge tray. This technology is the same as the one disclosed in the above-mentioned Japanese Unexamined Patent Publication No. 7-175279 in regard to that the double-sided document is inverted on the discharge tray in order to read the second surface.
In reading the double-sided document according to the technology of the above-mentioned Japanese Unexamined Patent Publication No. 7-175279, however, the document must be passed through the document reader unit three times when it is attempted to stack the documents on the discharge tray so that the pages are in order. In the third time of passage, however, the document simply passes through the document reader unit without being read out, but to cause the document reader unit to be fouled which then causes the image that is read out to become defective and makes it necessary to clean the document reader unit frequently. After the document is read out, the document reader unit is ready to read a next document. In order to sequentially discharge the documents, however, the documents must be passed through the document reader unit though they need not be really read out. Even after the document is read out, therefore, it is not allowed to feed the next document, making it difficult to increase the speed for reading the document. Moreover, the document must be conveyed over an increased distance requiring an extended period of time for conveying the document; i.e., efficiency for forming image decreases, and the document is likely to be damaged due to the pressure and friction created during the conveyance. Moreover, the document is inverted on the discharge tray. Therefore, the user may erroneously regard the document discharged on the discharge tray for being inverted to be the document that is discharged after the completion of the processing and may take it out. In such a case, the document processing machine comes into a halt due to defective conveyance of the paper. Furthermore, the image-forming machine that is forming images on both surfaces is also brought into a halt in the middle of image forming; i.e., the whole machine is halted.
According to the technology disclosed in Japanese Unexamined Patent Publication No. 8-133551, the double-sided document is inverted on the discharge tray like that of the above-mentioned Japanese Unexamined Patent Publication No. 7-175279 giving rise to the occurrence of a problem in that the document processing machine is halted due to defective conveyance of the paper and the whole image-forming apparatus is brought into a halt. Moreover, a plurality of inversion processing units are necessary requiring drive transmission mechanisms for the plurality of inversion rollers and control mechanisms therefor. Accordingly, the apparatus becomes complex and bulky necessitating a complex control mechanism.